gedenke, I too have a nice sporterized Arisaka 6.5X257, evidently done by an experienced gunsmith. Got it a few months ago. Rifling appears worn in mine, but actually the Japanese beveled rifling (polygonal?), so no sharp edges on them. However, my lands are heavily frosted/rough. I've tried different loads, but no luck improving my groups and I'd be happy to get 2" groups at 100 yards with mine. LOL, mine throws the bullets all over the target, with no real pattern to them. Looks like I have a bad scope, but after using 2 different scopes that work well on other rifles that is not the problem.
Read a post on another site recently by a guy with a 6.5 Arisaka which also shot unsatisfactory. He slugged his barrel and discovered it was not a 6.5 (.264). He discovered it was closer to the 6.5 Italian Carcano, which is .267 I think, not sure. Of course, he may have a Carcano rifle Japanese bought from Italians to use.
As for me, I've resigned myself to same decision you have made, need to rebarrel, but do I want to put any more money in my Arisaka. I use a retired machinest for my gunsmithing, something he has done on the side for about 40 years and is very capeable at it. His work is very reasonable in price, but he will not rebarrel an Arisaka. Just a quirk on his part, works on what he wants to. So, I'm considering buying another Marlin XL or XS rifle for around $300.
So, with the strenght of the Arisaka action and a barrel I was not worried about ruining, I tried an experiement with an engine crankcase additive that has teflon in it. Stuff looks like STP and it is called Greased Lightning and bottle of it costs me $6. I took a golf tee, dipped tip in the additive and then applied inside muzzle bore and then ran a cloth patch down bore to spread the stuff. Shot once, reswabbed additive in bore, shot again and etc. Repeated this for about 10 shots. My idea was for the teflon to become imbedded in the frosting of the lands, smoothing them out. Yea, crazy idea I know. But there is a rifle bore product called Gun Juice which I have read has teflon in it. Use same procedure with it as I used: apply, shoot, apply again and etc. Happened to be in an autoparts store one day, discovered the Greased Lightning product with teflon and said Why Not. Darn if this stupid idea didn't improve my Arisakas accuracy, groups tightened. Took rifle again to range to shoot. Shot about 6 rounds, not using the Greased Lightening. I fired two more rounds with the Greased Lightning in the bore. LOL, these two shots actually overlapped on my 100 yard target. I never had two consecutive bullets overlap each other with this rifle and while they were below the bullseye, they were both on the vertical center line of the bullseye. It was my last two 6.5X257 reloads, so don't know if it was just sure luck like I suspect. Two things I do know, bore is darker and it is easier to clean after the Greased Lightning.